Gold plating



United States Patent 3,156,635 GOLD PLATING Donald Gardner Foullre,Watchung, Piainfield, N.J., assignor to Sci-Rex Corporation, Nutley,N.J., a corporation of New Jersey N0 Drawing. Filed Aug. 23, 1%1, Ser.No. 133,301 Claims. (Cl. 20446) This invention is concerned with thedeposition of gold and specifically with the deposition of gold from abath relatively free of organic compounds, resulting in minimumcodeposition of organic materials with the gold.

From the early beginning of gold plating until very recently goldplating has been done from a cyanide electrolyte almost exclusively.Some plating was done from the acid trivalent gold complex, HAuCl butthis solution is highly corrosive and not at all suitable for productionwork. More recently gold has been plated from a bath on the acid side ofthe pH range, pH 3.5-6.0, to which was added considerable amounts oforganic acids and their salts. Although this formulation circumventedthe ageold problem of eliminating the build-up of undesirable cyanidedecomposition products characteristic of the conventional hot cyanidegold baths, obviously considerable organic material is present in thesolution, some of which may co-deposit with the gold.

This invention is based on the discovery that it is possible to obtain24-carat gold deposits essentially free of organic inclusions byemploying a gold bath containing potassium gold cyanide and conductingsalts consisting of phosphates or phosphites adjusted to a pH of 36.Gold deposition from such solutions proceeds at an efficiency of lessthan 100 percent resulting in some loss of hydrogen with the result thatthe phosphorous acidphosphite solution is somewhat more suitable from abuffer standpoint. Polyphosphoric acids may also be used although theygradually convert to orthophosphate.

The deposits obtained from this solution are not bright, being yellowmatte to semi-bright, but they are smooth and of high purity.

An object of this invention, therefore, is to provide an acid goldplating bath which is low in organic material so that there is littletendency to codeposit organic compounds with gold.

A second object of this invention is to provide a gold plating bathwhich is low in cyanide, thereby minimizing the tendency to build up aconcentration of cyanide break-down products in the bath. Obviously,some alkali cyanide will develop in the bath over a period of time, butat a pH of 36 the concentration is maintained at a low value. However, Ihave found that the introduction of hydrazine or substituted hydrazinestends to prevent this cyanide build-up and to provide improvedelectrodeposits.

The hydrazine compounds which may be employed are comprised of the watersoluble hydrazine salts which is the sulfate, N H .H SO thehydrochloride, N H .2HCl, etc., the water soluble organic hydrazine,hydrazono, and hydrazo compounds, etc. Approximately /2 to 20 g./l. ofthe hydrazine or hydrazine compound are satisfactory in preventing thebuild-up of cyanide products.

The gold content of the bath is not very critical but very satisfactorydeposits are obtained with from 4 to 36 g./l. of soluble gold salt.

3,156,635 Patented Nov. 10, 1964 ice The phosphate or phosphite anionsare present in the bath in the proportion of about 5 to 200 g./l. Thephosphate and phosphate anions, may be added to the bath as acids inwhich case, the pH is adjusted to the range of 36 by the addition ofalkali.

Examples of the method of carrying out this invention are as follows:

Example 1 A bath of the following composition was prepared:

KAu(CN) 12 KH PQ, 30

dm. The efficiency was 120 mg./ amp minute.

Example 2 A bath was made up as follows:

KAu(CN) g./l 12 Phosphorous acid (30%) ml 200 KOH to pH=3.5.

Brass panels plated in this bath were similar in appearance to thoseobtained from the phosphoric acid phosphate bath. The addition of methylhydrazine in one case and dimethyl hydrazine in another case gavebrighter deposits. Potassium phosphite could be substituted for theequivalent amount of KOH and phosphorous acid to make this bath.

Example 3 A bath was made up according to Example 1, except that only 2g./l. of gold (as sodium gold cyanide) were added. Satisfactory depositswere obtained up to 0.6 amp/dm. Increasing the phosphate content to 200g./l. had no observable effect on the deposit, but the voltage dropacross the cell was lowered.

Example 4 A bath was prepared as follows:

G./l. KAu(CN) 12 Polyphosphoric acid KOH to pH=3.5.

Deposits similar to those in Examples 1 and 2 were obtained, except thedeposits were brighter. Increasing the gold to 24 g./l. extended thepermissible current density range upwards, at a pH of 6.0 the depositscontinued to be ductile and were somewhat brighter.

The features and principles underlying the invention described above inconnection with specific exemplifications will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims shall not be limited to any specific feature ordetails thereof.

I claim:

1. The method of electrodepositing gold which coma prises electrolyzingan aqueous solution containing an alkali metal gold cyanide in theamount of 4 g./l. to 36 g./l., a nitrogen compound selected from thegroup consisting of hydrazine, water soluble substituted hydrazinecompounds and salts of hydrazine, and a combination of acidic anionsselected from the group consisting of phosphoric, phosphorous andpolyphosphoric acid anions and sufiicient alkali to give a pH of3.0-6.0.

2. The process as claimed in claim 1 in which the acid anion isphOSphate anion and is present in the proportion of about 5 to 200 g./l.

3. The process as claimed in claim 1 in which the acid anion isphosphite anion and is present in the proportion of about 5 to 200 g./l.

4. A plating solution for plating approximately 24k 15 selected from thegroup consisting of phosphoric, phosphorous and polyphosphoric acidanions,

sufficient alkali to provide a pH of 3 to 6 and an amount up to about 20g./l. of a hydrazine compound selected from the group consisting ofhydrazine, hydrazine salts and Water soluble substitution products ofhydrazine. 5. A plating solution as claimed in claim 4 comprising 1 to10 g./l. of hydrazine sulfate.

References Cited in the file of this patent UNITED STATES PATENTS2,176,668 Egeberg et a1 Oct. 17, 1939 2,654,702 De Long Oct. 6, 19532,812,299 Volk Nov, 5, 1957 2,978,390 Atwater et al Apr. 4, 1961

1. THE METHOD OF ELECTRODEPOSITING GOLD WHICH COMPRISES ELECTROLYZING ANAQUEOUS SOLUTION CONTAINING AN ALKALI METRAL GOLD CYANIDE IN THE AMOUNTOF 4 G/1 TO 36 G./L., A NITROGEN COMPOUND SELECTED FROM THE GROUPCONSISTING OF HYDRAZINE, WATER SOLUBLE SUBSITUTED HYDRAZINE COMPOUNDSAND SALTS OF HYDRAZINE, AND A COMBINATION OF ACIDIC ANIONS SELECTED FROMTHE GROUP CONSISTING OF PHOSPHORIC, PHOSPHOROUS AND POLYPHOSPHORIC ACIDANIONS AND SUFFICIENT ALKALI TO GIVE A PH OF 3.0-6.0